Press separator for fuel cell

ABSTRACT

A press separator made of a stainless steel sheet, for providing excellent corrosion resistance and electric conductivity by a combination of a passive coating and a deposition of boride or boron carbide, restriction corrosion without separation or coming-off, by press forming, of depositions, and ensuring an extended service. A stainless steel sheet, containing 0.005-1.5 wt. %, of B and having deposited on the surface thereof at least one kind out of M 23 (C, B) 6  type boron carbide, and M 2 B type and MB type borides, is press-formed in a corrugated shape having continuous irregularities, with angles of bent portions formed by bending or unbending by press forming being set at at least 15 degrees and an outward bending R-value at up to 1 mm.

TECHNICAL FIELD

The present invention relates to a separator for forming a gas passagein a solid high polymer fuel cell, and more particularly, relates to apress separator for a fuel cell formed in continuous corrugations bypress forming of a stainless steel plate.

BACKGROUND ART

A solid high polymer fuel cell is formed by laminating positive andnegative electrode catalyst layers (cathode and anode) on both sides ofan electrolyte membrane made of ion exchange resin or the like, andfurther laminating gas diffusion layers on these electrode catalystlayers to form an electrode structure, which is called a unit cell.Plural unit cells are laminated on both sides of a separator, and apractical fuel cell stack is formed. The separator is made of a materialhaving an electron transmitting function, and has multiple gas passagesformed like grooves for independently circulating fuel gas usinghydrogen and oxidizer gas such as oxygen or air, and is placed betweenunit cells in a state contacting with the gas diffusion layer.

In such fuel cells, for example, by circulating hydrogen gas as a fuelgas in the gas passage of the separator at the negative electrode side,and circulating oxidizing gas such as oxygen or air in the gas passageof the separator at the positive electrode side, an electrochemicalreaction takes place, and electricity is generated. During generation ofelectricity, the gas diffusion layer transmits electrons generated byelectrochemical reaction between the electrode catalyst layer and theseparator, and diffuses fuel gas and oxidizing gas at the same time. Theelectrode catalyst layer at the negative electrode side induces achemical reaction in the fuel gas, and generates protons and electrons,while the electrode catalyst layer at the positive electrode sideproduces water from oxygen, protons, and electrons, and the electrolytemembrane transmits protons ionically. Thus, electrical power is drawnout through the positive and negative electrode catalyst layers.

Hitherto, the separator was mainly made of graphite material, and thegas passages were formed by cutting grooves. Graphite materials includegas impermeable graphite having resin such as phenol resin impregnatedin baked isotropic graphite, amorphous carbon having resin such asphenol resin baked after forming, and composite material made of resinand graphite. These graphite materials are high in hardness, and it wasdifficult to form gas passages, or mechanical strength and impactresistance were poor.

In light of such problems, recently, it has been proposed to use newmaterials that can overcome the problems of the graphite materials, suchas press-formed materials of thin metal plates of aluminum, titanium,stainless steel, or the like. Among these, stainless steel has a passivefilm on the surface and is superior in corrosion resistance. However,when the stainless steel is used in the separator of a fuel cell,catalyst poisoning or conductivity reducing of electrode membrane may becaused by eluting ions. Moreover, since the electrical resistance of thepassive film is high, the contact resistance increases at the contactinterface of the separator and the electrode structure.

As means for solving these problems, a separator made of gold-platedstainless steel was proposed in Japanese Patent Application Laid-openNo. 10-228914. It has also been attempted to enhance the corrosionresistance and conductivity by precipitating conductive boride or boroncarbide from inside stainless steel, and exposing the precipitates onthe surface together with the passive film.

Of these conventional means of solution, the former method incurs a veryhigh manufacturing cost. Alternatively, if the gold plating is exposedto friction by vibration or the like, the gold plating is likely to peeloff at the interface with the stainless steel, and it is not suited tolong-term use. Moreover, if there is a pin hole or other defect,corrosion originates therefrom. In the latter means, on the other hand,the material becomes brittle due to precipitates appearing on thesurface, and when bent by press forming, the precipitates separate orfall off from the bent portion, and corrosion is initiated from the falloff marks, and this is also not suited to long-term use.

DISCLOSURE OF THE INVENTION

It is hence an object of the invention to provide a press separator forfuel cell capable of obtaining superior corrosion resistance andconductivity by combination of passive film and precipitates of borideor boron carbide, suppressing occurrence of corrosion without causingseparation or fall off of precipitates due to press forming, and whichwithstands long-term use.

The invention is characterized by using a stainless steel platecomprising B by 0.005 to 1.5 wt. %, with at least one of M₂₃(C, B)₆ typeboron carbide, M₂B type, and MB type borides precipitating on thesurface, being press formed in continuous corrugations, in which theangle of a bent portion formed by folding or elongating in press formingis 15 degrees or more, and the outer bending radius R is 1 mm or less.

According to the separator of the invention, many grooves formed on thesurface and reverse sides by press forming corrugations are used as gaspassages of fuel gas or oxidizing gas. In the separator of theinvention, since at least one type of the precipitates of boron carbideand borides is exposed on the surface, in addition to the high corrosionresistance realized by the passive film on the surface which is one ofthe characteristics of stainless steel, the corrosion resistance isfurther enhanced, and ion elution amount is reduced at the same time,and a high conductivity is obtained. Furthermore, generation of harmfulions and products is suppressed by the passive film and precipitates,and the constituent parts of the fuel cell such as electrolyte film orelectrode catalyst layer, or piping and other parts are not damaged bydischarge of such harmful substances.

Precipitates render materials brittle as mentioned above, and when bentand folded in press forming, precipitates may separate or fall off fromthe bent portion, and corrosion may be initiated from the fall offmarks. In the invention, however, since B is contained by 0.005 to 1.5wt. %, precipitates are prevented from separating or falling off fromthe bent portion by this defined content.

Boron is an important element of conductive inclusions precipitated onthe surface, and 0.005 wt. % or more is required from the viewpoint ofsatisfying the necessary precipitate amount to obtain the contactresistance necessary for the separator. If it exceeds 1.5 wt. %,however, the precipitate amount is excessive, and cracks or gaps may beformed, if not reaching the state of separation or fall off, on theouter surface of the bent portion formed by press forming, and corrosionmay be initiated from such defect. Therefore, the content of B isdefined to be in a range of 0.005 to 1.5 wt. %.

Gas passages of the separator of the invention are formed as grooves inthe surface and reverse sides of a stainless steel plate by pressforming in corrugations, and the angle of the bent portion for forminggas passages is defined to be 15 degrees or more, and the outer bendingradius R is 1 mm or less. FIGS. 1A and B show a partial section of theseparator obtained by press forming of a stainless steel plate incorrugations. A separator 1 in FIG. 1A has a gas passage 1 b formed inan isosceles triangle in which the angle θ of a bent portion 1 a is 90degrees. A separator 2 in FIG. 1B has a gas passage 2 b formed in atrapezoidal form in which the angle θ of a bent portion 2 a is 45degrees. In the invention, the radius of curvature of the outer side ofthe bent portion is the outer bending radius R.

Fuel gas or oxidizing gas flows in the gas passage of the separator, butsince the gas is consumed when contacting with the electrode structure,the gas passage is required to have a certain depth in order to maintaina necessary flow rate. From the viewpoint of the section of gas passage,a certain height (depth) is required against the width of the gaspassage. Supposing the width of the section to be W, the maximum depthformed at the angle θ of a bent portion is 0.5 W tan θ, and thesectional area is the maximum at this time. That is, assuming the ratioof the width and depth of section at this time, 0.5 W tan θ/W=0.5 tan θ,to be a parameter, the depth of the gas passage can be determined byapplying this parameter.

FIG. 3 shows results of measurement of generated voltages at 0.4 A/cm²power generation of a unit cell in fuel cells, in which a 0.2 mm thickstainless steel of the composition of the invention is press-formed at aconstant 0.5 mm of the outer bending radius R of bent portion whilevarying the angle of the bent portion to form separator and a fuel cellstack is formed by using the separators. As is understood from thisgraph, when the angle of the bent portion is 15 degrees or more, thepower generation efficiency is very high as compared with the angle ofless than 15 degrees. Hence, in the invention, the angle of the bentportion for forming the gas passage is defined to be 15 degrees or more.

The gas passages are required to have proper characteristics to allowgases to flow smoothly so that the fuel gas and oxidizing gas may besufficiently supplied into the electrode structure facing the gaspassages to assure a specified power generation efficiency. However, asshown in FIG. 2, a slight gap (shaded area in FIG. 2) is formed betweenthe outer side of the bent portion 3 a of the separator 3 and theelectrode structure 10 because the outer surface of the bent portion 3 ais a curved surface, and the gas tends to be stagnant in this gap. Thegas is supplied sufficiently into the electrode structure by minimizingthis gap.

FIG. 4 shows results of measurement of generated voltage at 0.4 A/cm²power generation of a unit cell in fuel cells, in which a 0.2 mm thickstainless steel of the composition of the invention is press-formed at aconstant 45 degrees of the bent portion while varying the outer bendingradius R of bent portion to form a separator and a fuel cell stack isformed by using the separators. As is understood from this graph, whenthe outer bending angle R is 1 mm or less, the power generationefficiency is very high as compared with the case of over 1 mm. Hence,in the invention, the outer bending radius R of the bent portion forforming the gas passage is defined to be 1 mm or less.

The invention is also characterized by using an austenitic stainlesssteel comprising B: 0.005 to 1.5 wt. %, C: 0.15 wt. % or less, Si: 0.01to 1.5 wt. %, Mn: 0.01 to 2.5 wt. %, P: 0.035 wt. % or less, S: 0.01 wt.% or less, Al: 0.001 to 0.2 wt. %, N: 0.3 wt. % or less, Cu: 0 to 3 wt.%, Ni: 7 to 50 wt. %, Cr: 17 to 30 wt. %, Mo: 0 to 7 wt. %, and balanceof Fe and inevitable impurities, with contents of Cr, Mo, and Bsatisfying the following formula:Cr(wt. %)+3×Mo(wt. %)−2.5×B(wt. %)≧17,precipitating at least one of M₂₃(C, B)₆ type boron carbide, M₂B type,and MB type borides on the surface, and being press formed in continuouscorrugations, in which the angle of a bent portion formed by folding orelongating in press forming is 15 degrees or more and the outer bendingradius R is 1 mm or less.

The reasons for setting the numerical values of the contents of theelements except B are explained below.

C: 0.15 wt. % or Less

The content of C is preferred to be as low as possible in order toassure the cold toughness and ductility to satisfying press formingperformance suited to mass production, and hence it is defined to be0.15 wt. % or less in the invention.

Si: 0.01 to 1.5 wt. %

Si is effective as a deoxidizing element, but if it is less than 0.01wt. %, the deoxidizing effect is not sufficient, or if it exceeds 1.5wt. %, the ductility is reduced and the press forming performance isimpeded. Hence, the content of Si is defined to be in a range of 0.01 to1.5 wt. %

Mn: 0.01 to 2.5 wt. %

Mn is necessary as a deoxidizing element, and is also added as a balanceadjusting element of Ni. It also functions to solidify mixed S which isan inevitable impurity as a sulfide of Mn. These functions are exhibitedwhen the content of Mn is 0.01 wt. % or more, but if it exceeds 2.5 wt.%, the ion elution amount increases, and in particular, when theelectrolyte membrane is a sulfonic acid compound, it bonds with asulfonic acid radical, and the ion conductivity of the electrolytemember is lowered. Hence, the content of Mn is defined to be in a rangeof 0.01 to 2.5 wt. %.

P: 0.035 wt. % or Less

P is an element inevitably mixed in, and its content should be as low aspossible. Considering that the precipitate (inclusion) containing P maybe the origin of corrosion under the fuel cell condition, the content ofP is defined to be at 0.035 wt. % or less.

S: 0.01 wt. % or Less

Due the same reasons as for P, the content of S is defined to be at 0.01wt. % or less.

Al: 0.001 to 0.2 wt. %

Al is added in the steel melting stage as a deoxidizing element, and iscontained in a range of 0.001 to 0.2 wt. %. Since B in the steel is anelement having a strong bonding power with oxygen in the molten steel,the oxygen concentration must be lowered by the deoxidizing action ofAl.

N: 0.3 wt. % or Less

Due to the same reasons as for C, the content of N is defined to be at0.3 wt. %.

Cu: 0 to 3 wt. %

As required, Cu is contained at 3 wt. % or less. When a proper amount ofCu is contained, passivation is promoted, and it is effective to preventelution of metal in the separator environment. The content is preferredto be 0.01 wt. % or more, but when it exceeds 3 wt. %, the processingefficiency in hot process is lowered, and mass production is difficult.Hence, the content of Cu is defined to be in a range of 0 to 3 wt. %

Ni: 7 to 50 wt. %

Ni is an important element for making austenitic metallographically. Themanufacturing property, corrosion resistance, and forming performanceare assured by making austenitic. When the content of Ni is less than 7wt. %, it is difficult to form an austenitic texture, and if it exceeds50 wt. %, it becomes too costly. Hence, the content of Ni is defined tobe in a range of 7 to 50 wt. %. Meanwhile, Ni is slightly contained inM₂B type boride.

Cr: 17 to 30 wt. %

The higher the content of Cr, the higher the corrosion resistance, buttoughness and ductility at ordinary temperatures are reduced.Considering the balance of corrosion resistance and toughness andductility, the content of Cr is defined to be in a range of 17 to 30 wt.% in the invention.

Mo: 0 to 7 wt. %

The higher the content of Mo, the higher the corrosion resistance;however, the material becomes brittle. So as not to be brittle, in theinvention, the content of Mo is defined to be in a range of 0 to 7 wt. %Cr(wt. %)+3×Mo(wt. %)−2.5×B(wt. %)≧17,

Since B consumes Cr and Mo in the stainless steel to produce borides andboron carbides, the contents of Cr and Mo as corrosion preventionimproving elements contained in the base material are reduced, and thecorrosion resistance of the base material is reduced, and hence thisformula is defined.

In other aspects, the invention is characterized by using a ferriticstainless steel comprising B: 0.005 to 1.5 wt. %, C: 0.15 wt. % or less,Si: 0.01 to 1.5 wt. %, Mn: 0.01 to 1.5 wt. %, P: 0.035 wt. % or less, S:0.01 wt. % or less, Al: 0.001 to 0.2 wt. %, N: 0.035 wt. % or less, Cu:0 to 1 wt. %, Ni: 0 to 5 wt. %, Cr: 17 to 36 wt. %, Mo: 0 to 7 wt. %,and balance of Fe and inevitable impurities, with the contents of Cr,Mo, and B satisfying the following formula:Cr(wt. %)+3×Mo(wt. %)−2.5×B(wt. %)≧17,precipitating at least one of M₂₃(C, B)₆ type boron carbide, M₂B type,and MB type borides on the surface, and being press formed in continuouscorrugations, in which the angle of a bent portion formed by folding orelongating in press forming is 15 degrees or more and the outer bendingradius R is 1 mm or less. The contents of Mn, N, Cu, and Ni in thisseparator are slightly different from the contents in the separatorcomposed of the austenitic stainless steel mentioned above, but thereasons for setting the upper and lower limits of these numerical valuesare the same as explained above.

Furthermore, in the press separator for fuel cells of the invention,stainless steel plates including austenitic stainless steel plates andferritic stainless steel plates are preferred to be steel platesfinished by bright annealing, and by this bright annealing process,formation of a de-B layer can be prevented in the surface layer whichcannot be prevented from oxidation in air, and decrease in the number ofconductive inclusions exposed after pickling can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partial sectional view conceptually showing a separator ofthe invention, and FIG. 1B is a partial sectional view conceptuallyshowing a separator in another embodiment of the invention.

FIG. 2 is a diagram showing a gap allowing a stagnant flow of gas, beingformed between bent portion of a separator and an electrode structure.

FIG. 3 is a diagram showing the relationship between angle of bentportion forming a gas passage of a separator and generated voltage of afuel cell.

FIG. 4 is a diagram showing the relationship between outer bendingradius R of bent portion forming a gas passage of a separator andgenerated voltage of a fuel cell.

FIG. 5 is a diagram showing the correlation of B content, outer bendingradius R, and corrosion state of bent portion of a separator composed ofan austenitic stainless steel plate.

FIG. 6 is a diagram showing the correlation of B content, angle of bentportion, and corrosion state of bent portion of a separator composed ofan austenitic stainless steel plate.

FIG. 7 is a diagram showing the correlation of B content, outer bendingradius R, and corrosion state of bent portion of a separator composed ofa ferritic stainless steel plate.

FIG. 8 is a diagram showing the correlation of B content, angle of bentportion, and corrosion state of bent portion of a separator composed ofa ferritic stainless steel plate.

FIG. 9A is a plan of a separator fabricated in Examples, and FIG. 9B isits sectional view.

FIG. 10 is a sectional view of a fuel cell stack fabricated in Examples.

FIG. 11 is a diagram showing results of measurement of contactresistance and passive state holding current density at 0.9 V of aseparator composed of an austenitic stainless steel plate executed inExamples.

FIG. 12 is a diagram showing changes as time passed of contactresistance of a separator composed of an austenitic stainless steelplate executed in Examples.

FIG. 13 is a diagram showing changes as time passed of current densityof a separator composed of an austenitic stainless steel plate executedin Examples

FIG. 14 is a diagram showing results of measurement of contactresistance and passive state holding current density at 0.9 V of aseparator composed of a ferritic stainless steel plate executed inExamples.

FIG. 15 is a diagram showing changes as time passed of contactresistance of a separator composed of a ferritic stainless steel plateexecuted in Examples.

FIG. 16 is a diagram showing changes as time passed of current densityof a separator composed of a ferritic stainless steel plate executed inExamples.

BEST MODE FOR CARRYING OUT THE INVENTION

The effects of the invention are demonstrated by presenting Examplesbelow.

(1) Relationship Between B Content and Outer Bending Radius R(Austenitic Stainless Steel)

Various separators of different combinations of B content and outerbending radius were fabricated by using 0.2 mm thick austeniticstainless steel plates with the content of B variable in a range of 0 to2 wt. % and the contents of the other elements within the range of theinvention, and by press forming with the bending angle of the bentportion constant (15 degrees) and the outer bending radius R rangingfrom 0.2 to 1.6 mm. Using each separator, a fuel cell was formed, aspecified gas was circulated in gas passages to generate powercontinuously for 3000 hours, and separation, fall off, and corrosion ofthe bent portion of the separator were observed. FIG. 5 shows theresults, in which the ◯-mark shows a separator in a sound state freefrom corrosion originated from separation or fall off mark at thesurface, and the X-mark indicates a corroded separator.

(2) Relationship Between B Content and Angle of Bent Portion (AusteniticStainless Steel)

Likewise, various separators of different combinations of B content andangle of bent portion were fabricated by using 0.2 mm thick austeniticstainless steel plates with the content of B variable in a range of 0 to2 wt. % and the contents of the other elements within the range of theinvention, and by press forming with the outer bending radius of thebent portion constant (1 mm) and the angle of the bent portion rangingfrom 0 to 120 degrees. Using each separator, a fuel cell was formed, aspecified gas was circulated in gas passages to generate powercontinuously for 3000 hours, and separation, fall off, and corrosion ofthe bent portion of the separator were observed. FIG. 6 shows theresults, in which the evaluation is indicated by ◯-marks and X-marks inthe same way as in FIG. 5.

According to FIG. 5, if the outer bending radius R is defined to be at 1mm or less, corrosion occurs unless the B content is 1.5 wt. % or less.According to FIG. 6, if the angle of the bent portion is defined to beat 15 degrees or more, similarly, the B content must be 1.5 wt. % orless. Therefore, in the separators made of austenitic stainless steelplates of the invention, in order to prevent separation and fall off ofprecipitates of borides or boron carbides due to precipitation bycontaining B and corrosion originating from fall off marks, theessential conditions are the B content of 1.5 wt. % or less, the outerbending radius R of 1 mm, and angle of the bent portion of 15 degrees ormore. However, the content of B must be 0.005 wt. % from the viewpointof satisfying the necessary precipitation amount for assuring thecontact resistance necessary for the separator.

(3) Relationship Between B Content and Outer Bending Radius R (FerriticStainless Steel)

Various separators of different combinations of B content and outerbending radius were fabricated by using 0.2 mm thick ferritic stainlesssteel plates with the content of B variable in a range of 0 to 2 wt. %and the contents of the other elements within the range of theinvention, and by press forming with the bending angle of the bentportion constant (15 degrees) and the outer bending radius R rangingfrom 0.2 to 1.6 mm. Using each separator, a fuel cell was formed, aspecified gas was circulated in gas passages to generate powercontinuously for 3000 hours, and separation, fall off, and corrosion ofthe bent portion of the separator were observed. FIG. 7 shows theresults, in which the evaluation is indicated by ◯-marks and X-marks inthe same way as in FIG. 5.

(4) Relationship Between B Content and Angle of Bent Portion (FerriticStainless Steel)

Likewise, various separators of different combinations of B content andangle of bent portion were fabricated by using 0.2 mm thick ferriticstainless steel plates with the content of B variable in a range of 0 to2 wt. % and the contents of the other elements within the range of theinvention, and by press forming with the outer bending radius of thebent portion constant (1 mm) and the angle of the bent portion rangingfrom 0 to 120 degrees. Using each separator, a fuel cell was formed, aspecified gas was circulated in gas passages to generate powercontinuously for 3000 hours, and separation, fall off, and corrosion ofthe bent portion of the separator were observed. FIG. 8 shows theresults, in which the evaluation is indicated by ◯-marks and X-marks thesame way as in FIG. 5.

According to FIG. 7, if the outer bending radius R is defined to be at 1mm or less, corrosion occurs unless the B content is 1.5 wt. % or less.According to FIG. 8, if the angle of the bent portion is defined to beat 15 degrees or more, similarly, the B content must be 1.5 wt. % orless. Therefore, in the separators made of ferritic stainless steelplates of the invention, in order to prevent separation and fall off ofprecipitates of borides or boron carbides due to precipitation bycontaining B and corrosion originating from fall off marks, theessential conditions are the B content of 1.5 wt. % or less, the outerbending radius R of 1 mm, and angle of the bent portion of 15 degrees ormore. However, the content of B must be 0.005 wt. % from the viewpointof satisfying the necessary precipitation amount for assuring thecontact resistance necessary for the separator.

(5) Difference in Performance by B Content (Austenitic Stainless Steel)

Using 0.2 mm thick austenitic stainless steels having the composition inExample 1 (within scope of the invention) and Comparative Example 1 (outof scope of the invention) shown in Table 1, separators 4 shown in FIGS.9A and B were fabricated by press forming. As shown in FIG. 9B, the gaspassage 4 b of the separator 4 was trapezoidal, the angle of the bentportion 4 a was 45 degrees, and the outer bending radius R was 0.3 mm.In these separators, the contact resistance and passive state holdingcurrent density at 0.9 V were measured. Results of measurement arerecorded in FIG. 11. The contact resistance is a through-resistancemeasured by applying a surface load of 5 kgf cm² on two overlaid pliesof separators (anode side and cathode side) 4, using a resistance meter.The passive state holding current density refers to the current densitycorresponding to the rate of corrosion when the oxide forming speed ofthe stainless steel of the base material becoming an oxide and the speedof the surface oxide film being melted to become ions are equalized,that is, when the thinness of the oxide film no longer changes, and thiscurrent density was measured by a constant potential polarization test.TABLE 1 Element content unit: wt. % C Si Mn P S Cu Ni Cr Mo N Al B Cr +3Mo − 2.5B Example 1 0.018 0.65 1.02 0.028 0.0078 0.25 8.4 18.82 — 0.0250.015 0.12  18.52 Example 2 0.018 0.65 1.02 0.028 0.0078 0.25 0.21 18.82— 0.025 0.015 0.12  18.52 Comparative 0.019 0.12 0.08 0.013 0.0008 0.088.4 24.76 — 0.036 0.022 2.52* 18.46 Example 1 Comparative 0.019 0.120.08 0.013 0.0008 0.08 0.01 24.76 — 0.036 0.022 2.52* 18.46 Example 2*Value out of scope of the invention

Next, as shown in FIG. 10, using ten unit cells 20 composed of electrodestructures, a fuel cell stack was composed by laminating by interposingthe separator 4 in Example 1 among the unit cells 20. In the diagram,reference numeral 21 is a seal, reference numeral 22 is a currentcollector plate, and reference numeral 23 is a clamp plate for fixingthe laminated state of the fuel cell stack. On the other hand, using theseparator of Comparative Example 1, a fuel cell stack was similarlyfabricated. Using these fuel cells, power was generated, and the contactresistance from start of power generation until 3000 hours later atintervals of 500 hours, and the current density at 0.7 V powergeneration of unit cells were measured. Results of the measurements areshown in FIG. 12 and FIG. 13.

According to FIG. 11, as far as the contact resistance was concerned,there was no significant difference between Example 1 and ComparativeExample 1, but the passive state holding current density at 0.9 V wassubstantially higher in Comparative Example 1 as compared withExample 1. According to FIG. 12, only upon start of power generation,the contact resistance was low and similar in Example 1 and ComparativeExample 1, but Comparative Example 1 began to increase in the contactresistance from immediately after the start of power generation, andfurther increased as time passed. In contrast, in Example 1, the contactresistance remained at a low level and did not change in spite ofgenerating power for a long period. In addition, according to FIG. 13,only upon start of power generation, the current density was similar inExample 1 and Comparative Example 1, but Comparative Example 1 began toreduce in the current density from immediately after start of powergeneration, and further reduced as time passed. In contrast, in Example1, the current density remained at a low level and did not change inspite of power generation for a long period.

(6) Difference in Performance by B Content (Ferritic Stainless Steel)

Using 0.2 mm thick ferritic stainless steels having the composition inExample 2 (within the scope of the invention) and Comparative Example 2(out of the scope of the invention) shown in Table 1, separators werefabricated in the same way as in Example 1. In these separators, thecontact resistance and passive state holding current density at 0.9 Vwere measured in the same way as above. Results of measurement arerecorded in FIG. 14.

Next, in the same way as in Example 1, a fuel cell stack was formed byusing the separator of Example 2, and furthermore, a fuel cell wasformed by using the separator of Comparative Example 2. Using these fuelcells, power was generated, and the contact resistance from start ofpower generation until 3000 hours later at intervals of 500 hours, andthe current density at 0.7 V power generation of unit cells weremeasured. Results of measurement are shown in FIG. 15 and FIG. 16.

According to FIG. 14, as far as the contact resistance was concerned,there was no significant difference between Example 2 and ComparativeExample 2, but the passive state holding current density at 0.9 V wassubstantially higher in Comparative Example 2 as compared with Example2. According to FIG. 15, only upon start of power generation, thecontact resistance was low and was similar to that in Example 2 andComparative Example 2, but Comparative Example 2 began to increase inthe contact resistance from immediately after start of power generation,and further increased as time passed. In contrast, in Example 2, thecontact resistance remained at a low level and did not change in spiteof power generation for a long period. In addition, according to FIG.16, only upon start of power generation, the current density was similarin Example 2 and Comparative Example 2, but Comparative Example 2 beganto decline in the current density from immediately after the start ofpower generation, and further declined as time passed. In contrast, inExample 2, the current density remained at a low level and did notchange in spite of power generation for a long period.

1. A press separator for a fuel cell made of a stainless steel platecomprising B by 0.005 to 1.5 wt. %, precipitating at least one of M₂₃(C,B)₆ type boron carbide, M₂B type, and MB type borides on the surface,and being press formed in continuous corrugations, wherein angle of abent portion formed by folding or elongating in press forming is 15degrees or more, and outer bending radius R is 1 mm or less. 2.(canceled)
 3. A press separator for a fuel cell made of a ferriticstainless steel comprising B: 0.005 to 1.5 wt. %, C: 0.15 wt. % or less,Si: 0.01 to 1.5 wt. %, Mn: 0.01 to 1.5 wt. %, P: 0.035 wt. % or less, S:0.01 wt. % or less, Al: 0.001 to 0.2 wt. %, N: 0.035 wt. % or less, Cu:0 to 1 wt. %, Ni: 0 to 5 wt. %, Cr: 17 to 36 wt. %, Mo: 0 to 7 wt. %,and balance being Fe and inevitable impurities, with contents of Cr, Mo,and B satisfying the following formula:Cr(wt. %)+3×Mo(wt. %)−2.5×B(wt. %)≧17, precipitating at least one ofM₂₃(C, B)₆ type boron carbide, M₂B type, and M₂B type borides on thesurface, and being press formed in continuous corrugations, whereinangle of a bent portion formed by folding or elongating in press formingis 15 degrees or more, and outer bending radius R is 1 mm or less.
 4. Apress separator for a fuel cell made of a bright annealed stainlesssteel plate comprising B by 0.005 to 1.5 wt. %, precipitating at leastone of M₂₃(C, B)₆ type boron carbide, M₂B type, and MB type borides onthe surface, and being press formed in continuous corrugations, whereinangle of a bent portion formed by folding or elongating by press formingis 15 degrees or more, and outer bending radius R is 1 mm or less. 5.(canceled)
 6. A press separator for a fuel cell made of a brightannealed ferritic stainless steel comprising B: 0.005 to 1.5 wt. %, C:0.15 wt. % or less, Si: 0.01 to 1.5 wt. %, Mn: 0.01 to 1.5 wt. %, P:0.035 wt. % or less, S: 0.01 wt. % or less, Al: 0.001 to 0.2 wt. %, N:0.035 wt. % or less, Cu: 0 to 1 wt. %, Ni: 0 to 5 wt. %, Cr: 17 to 36wt. %, Mo: 0 to 7 wt. %, and balance being Fe and inevitable impurities,with contents of Cr, Mo, and B satisfying the following formula:Cr(wt. %)+3×Mo(wt. %)−2.5×B(wt. %)≧17, precipitating at least one ofM₂₃(C, B)₆ type boron carbide, M₂B type, and MB type borides on thesurface, and being press formed in continuous corrugations, whereinangle of a bent portion formed by folding or elongating in press formingis 15 degrees or more, and outer bending radius R is 1 mm or less.